The next generation wireless local area network (LAN) can provide a high speed multimedia service to terminals such as a notebook computer, a personal computer, and a portable digital assistant (PDA) at a transmission rate of 200 Mbps within a 100 m radius. Also, the next generation wireless LAN can provide a wireless network environment for Hot-spot, Office Networking, and home networking.
Lately, a wireless LAN technology has been popularly used to provide a wireless Internet service in predetermined public areas such as an airport, a hotel, and a cafe. Particularly, it has been enabled to form a wireless home network in home in a form of a wireless LAN system combined with a broadband wired subscriber loop network such as Asymmetry Digital Subscribe Loop (ADSL) in South Korea.
In 1999, the standards for 2.4 GHz band of IEEE 802.11b and for 5 GHz band of IEEE 802.11a were defined. After that, 802.11b systems have been commercialized and the wireless LAN has been popularized. At the initial stage, the wireless LAN was provided as a private network without a service provider. Since 2002, a public network service has been provided in predetermined nations such as South Korea and U.S.A. Lately, the range of the public network service has been continuously extending. Since products related to IEEE 802.11a and IEEE 802.11g have come out on a market, data can be transmitted at 54 Mbps.
Recently, an IEEE 802.11n task group has been discussing a pane for increasing the maximum throughput of a medium access control (MAC) layer higher than 100 Mbps rather than increasing the throughput of a physical layer.
In other words, not only the physical layer but also the MAC layer has been carefully studied to improve the throughput. In order to improve the throughput, a Multiple Input Multiple Output (MIMO) system for improving frequency efficiency using multiple antennas, a method for improving a bandwidth, and an adaptive modulation have been researched.
The MIMO system forms a plurality of independent fading channels at a transmitter using multiple antennas and transmits different signals through the multiple transmission antennas simultaneously, thereby significantly improving a data transmission speed. Accordingly, the MIMO system can transmit a further large amount of data even without expanding a frequency.
However, the MIMO system has a drawback that the MIMO system is weak to interference between symbols and frequency selective fading. In order to overcome the drawback, an Orthogonal Frequency Division Multiple (OFDM) scheme has been used together. The OFDM scheme is a modulation scheme most suitable for high speed data transmission. The OFDM scheme transmits data through a sub-carrier having a data transmission rate lower than one data row.
By combining the MIMO system with the OFDM system, the drawback of the MIMO system can be attenuated using the OFDM system without eliminating the advantageous of the MIMO system. The MIMO system generally includes N transmission antennas and M reception antennas, and a MIMO-OFDM system is a typical MIMO system employing an OFDM technology.
However, the MIMO system has following shortcomings. The MIMO system needs at least two or three transmission antennas and reception antennas. Also, the MIMO system cannot expands a bandwidth wider than 40 MHz and has a limited channel capacity such as a several mega transmission speed. Lately, a very high-throughput (VHT) task group has been discussing about a Giga level wireless communication system. It has been required to develop a system having a wide bandwidth in a high speed wireless LAN technology using the MIMO system.